Dick Dowden, a retired atmospheric/radio physicist at Otago University, who set
up a world-wide Lightning Locator system, responded.

Before and during big thunderstorms, air from the near the ground is sucked up to high altitudes.
These "updrafts" carry water vapour (steam) up to heights as much as 10,000 metres
(nearly three times as high as Mount Cook) in tropical regions. Long before the water
vapour gets that high it condenses out as cloud to make very tall thunderclouds.

There is always some electric charge floating around near the ground, where you and I live,
which is also carried up to these very high altitudes. Carrying this charge up increases
the voltage of the air containing this electric chage to about a million volts.

Have you seen a Van der Graaf machine at school? In this, a small charge (maybe from a
comb run though your hair or rubbed on your jumper) is carried up on a belt to the metal
sphere at the top of the Van der Graaf machine. So you can see that the updraft in a
thunderstorm is like the belt in a Van der Graaf machine.

When the cloud top voltage is a million volts, the air breaks down to make a huge
electrical discharge we see as lightning.